Planetary Defense: The Bennu Experiment

This artist's concept shows the Origins Spectral Interpretation Resource Identification Security - Regolith Explorer (OSIRIS-REx) spacecraft contacting the asteroid Bennu with the Touch-And-Go Sample Arm Mechanism or TAGSAM. The mission aims to return a sample of Bennu's surface coating to Earth for study as well as return detailed information about the asteroid and it's trajectory.Credits: NASA's Goddard Space Flight Center

On Dec. 3,
after traveling billions of kilometers from Earth, NASA's OSIRIS-REx spacecraft
reached its target, Bennu, and kicked off a nearly two-year, up-close investigation
of the asteroid. It will inspect nearly every square inch of this ancient clump
of rubble left over from the formation of our solar system. Ultimately, the
spacecraft will pick up a sample of pebbles and dust from Bennu's surface and deliver
it to Earth in 2023.

Generations of
planetary scientists will get to study pieces of the primitive materials that
formed our cosmic neighborhood and to better understand the
role asteroids may have played in delivering
life-forming compounds to planets and moons.

But it's not
just history that the mission to Bennu will help uncover. Scientists studying
the rock through OSIRIS-REx's instruments in space will also shape our future.
As they collect the most detailed information yet about the forces that move
asteroids, experts from NASA's Planetary Defense Coordination Office, who are responsible
for detecting potentially hazardous asteroids, will improve their predictions of
which ones could be on a crash-course with our planet.

Here is how the
OSIRIS-REx mission will support this work:

How scientists predict Bennu's whereabouts

About a third
of a mile, or half a kilometer, wide, Bennu is large enough to reach Earth's
surface; many smaller space objects, in contrast, burn up in our atmosphere. If
it impacted Earth, Bennu would cause widespread damage. Asteroid experts at the
Center
for Near-Earth Object Studies (CNEOS) at NASA's Jet Propulsion Laboratory
in Pasadena, California, project
that Bennu will come close enough to Earth over the next century to pose a 1 in
2,700 chance of impacting it between 2175 and 2196. Put another way,
those odds mean there is a 99.963 percent chance the asteroid will miss the
Earth. Even so, astronomers want to know exactly where Bennu is located at all
times.

Astronomers have
estimated Bennu's future trajectory after observing it several times since it
was discovered in 1999. They've turned their optical, infrared and radio
telescopes toward the asteroid every time it came close enough to Earth, about
every six years, to deduce features such as its shape, rotation rate and
trajectory.

"We know
within a few kilometers where Bennu is right now," said Steven
Chesley, senior research scientist at CNEOS and an OSIRIS-REx team member whose job
it is to predict Bennu's future trajectory.

Why Bennu's future trajectory predictions
get fuzzy

Scientists have
estimated Bennu's trajectory around the Sun far into the future. Their
predictions are informed by ground observations and mathematical calculations that
account for the gravitational nudging of Bennu by the Sun, the Moon, planets and
other asteroids, plus non-gravitational factors.

Given these
parameters, astronomers can predict the next four exact dates (in September of 2054,
2060, 2080 and 2135) that Bennu will come within 5 million miles (7.5 million
kilometers or .05 astronomical units) of Earth. That's close enough that Earth's
gravity will slightly bend Bennu's orbital path as it passes by. As a result, the
uncertainty about where the asteroid will be each time it loops back around the
Sun will grow, causing predictions about Bennu's future orbit to become increasingly
hazy after 2060.

In 2060, Bennu
will pass Earth at about twice the distance from here to the Moon. But it could
pass at any point in a 19-mile (30-kilometer) window of space. A very small
difference in position within that window will get magnified enormously in
future orbits and make it increasingly hard to predict Bennu's
trajectory.

As a result, when
this asteroid comes back near Earth in 2080, according to Chesley's
calculations, the best window we can get on its whereabouts is nearly 9,000
miles (14,000 kilometers) wide. By 2135, when Bennu's shifted orbit is expected
to bring it closer than the Moon, its flyby window grows wider, to nearly
100,000 miles (160,000 kilometers). This will be Bennu's closest approach to
Earth over the five centuries for which we have reliable calculations.

"Right now,
Bennu has the best orbit of any asteroid in our database," Chesley said. "And
yet, after that encounter in 2135, we really can't say exactly where it is
headed."

There's another
phenomenon nudging Bennu's orbit and muddying future impact projections. It's
called the Yarkovsky effect. Having nothing to do with gravity, the Yarkovsky
effect sways Bennu's orbit because of heat from the Sun.

"There are
a lot of factors that might affect the predictability of Bennu's trajectory in
the future, but most of them are relatively small," says William
Bottke, an asteroid expert at the Southwest Research Institute in
Boulder, Colorado, and a participating scientist on the OSIRIS-REx mission. "The one that's most sizeable is
Yarkvovsky."

This heat nudge
was named after the Polish civil engineer who first described it in 1901: Ivan
Osipovich Yarkovsky. He suggested that sunlight warms one side of a small,
dark asteroid and some hours later radiates that heat away as the asteroid
rotates its hot side into cold darkness. This thrusts the rock pile a bit,
either toward the Sun or away from it, depending on the direction of its
rotation.

In Bennu's case, astronomers
have calculated that the Yarkovsky effect has shifted its orbit about 0.18
miles (284 meters) per year toward the Sunsince 1999. In fact, it helped
deliver Bennu to our part of the solar system, in the first place, from the
asteroid belt between
Mars and Jupiter over billions of years. Now, Yarkovsky is complicating
our efforts to make predictions about Bennu's path relative to Earth.

Getting face-to-face with the asteroid
will help

The OSIRIS-REx
spacecraft will use its suite of
instruments to transmit radio tracking signals and capture optical
images of Bennu that will help NASA scientists determine its precise position
in the solar system and its exact orbital path. Combined with existing,
ground-based observations, the space measurements will help clarify how Bennu's
orbit is changing over time.

Additionally, astronomers
will get to test their understanding of the Yarkovksy effect on a real-life
asteroid for the first time. They will instruct the spacecraft to follow Bennu
in its orbit about the Sun for about two years to see whether it's moving along
an expected path based on gravity and Yarkovsky theories. Any differences
between the predictions and reality could be used to refine models of the
Yarkovsky effect.

But even more
significant to understanding Yarkovsky better will be the thermal measurements
of Bennu. During its mission, OSIRIS-REx will track how much solar heat radiates
off the asteroid, and where on the surface it's coming from-data that will help
confirm and refine calculations of the Yarkovsky effect on asteroids.

The spacecraft
also will address some open questions about the Yarkovsky theory. One of them, said
Chesley, is how do boulders and craters on the surface of an asteroid change
the way photons scatter off of it as it cools, carrying away momentum from the
hotter side and thereby nudging the asteroid in the opposite direction?
OSIRIS-REx will help scientists understand by mapping the rockiness of Bennu's
surface.

"We know
surface roughness is going to affect the Yarkovsky effect; we have models"
said Chesley. "But the models are speculative. No one has been able to test
them."

After the
OSIRIS-REx mission, Chesley said, NASA's trajectory projections for Bennu will
be about 60 times better than they are now.